Radio transmitting system



Sept. 13, 1938. E. H. ARMSTRONG 2,130,172

RADIO TRANSMITTING SYSTEM Filed Aug. 25, 1936 H mm 1NVENTOR. EdW/n H. Armsfronq. BY

ATTORNEY.

Patented Sept. 13, 1938 UNITED STATES PAT NT OFFICE RADIO TRANSMITTING SYSTEM Edwin H. Armstrong, New York, N. Y.

Application August 25, 1936, Serial No. 97,736

2Claims.

This invention relates to an improvement in the method of modulating the transmitter described in myU. S. Patent #1,941,068, whereby a more symmetrical frequency characteristic is obtained and whereby distortion in theradiated wave is reduced.

The invention is of particular importance where a wide range of modulating frequencies are employed such as in multiplex signaling, television and the like, because it is difiicult with the arrangement shown in the said patent to prevent asymmetrical phase shift at the higher frequencies of modulation.

Fig. I illustrates the arrangement for carrying out the invention. In this figure, i represents the master oscillator, 2 a bufier amplifier and 3 a transformer. In the secondary of this transformer are three parallel circuits, one containing the resistances 6 and i in series with each other, the other two containing respectively resistance, capacity, and inductance in series as shown. i2 is an amplifier whose grid is excited by the voltage developed across the resistance 1. I l and l are likewise amplifiers whose grids are excited respectively by the voltages existing across the capacity 9 and the inductance H. The plates of the tubes I4, l5 are connected in parallel with each other and arranged to have a common plate load with the tube I2 by means of the resistance l3. it represents an amplifier of the combined outputs of the tubes i2, i4 and l5. i1 is a band pass filter foreliminating undesired harmonics, I8 is a frequency multiplier and Mia represents the output of the system. l9 represents the input of the modulating system, 20 an amplifier, 2! a blocking condenser, 22, 23 the correction system, and 24 an amplifier which diiferentially modulates the screen grids of the amplifiers i4 and I5 thru the transformer 25. I

The method of operation of the system is as follows. The master oscillator I produces in the output circuit of the transformer 3 a current of the desired frequency. This current is split up into three parts; one to each branch of the parallel system. The inductance and capacity combinatioi s in the two branches containing reactance are made non-reactive for the oscillator frequency. The currents in the three branch circuits are therefore all in phase witheach other. The E. M. F. developed across the resistance 1 which is applied to the grid of the amplifier I2 is, consequently, in phase with this current. The E. M. F. developed across condenser 9 which is applied to the gridbf the modulator I5 is 90 ahead of the current and the E. M. F. developed rent. These two E. M. F.s electromotive forces difier from each other by 180. By making the resistances 4' and 5 equal to each other and the reactances of condenser 9 and inductance ll likewise equal to each other for the oscillator frequency the voltages applied to the grids of M and it may be made equal in amplitude and opposite in phase. Since the electromotive forces applied to the grids of the tubes i4 and I5 are equal in magnitude and opposite in phase it follows that the currents in the plate circuits of these tubes are equal and opposite in phase. Consequently in the absence of modulation the only current which flows thru the resistance i3 which is common to the three tubes I 2, l6 and I5 will be the current due to the amplifier Hi. This current will be in phase with the E. M. F. applied to the grid of the tube.

Suppose now that the potential of the screen grids of the modulator tubes I4 and I5 be changed, one being made more positive and the other being made more negative. More current will flow thru the tube whose screen grid potential has been rendered more positive and less current will flow thru the tube whose screen grid potential has been rendered less positive. Consequently the two currents no longer balance each other and the difierence will flow thru the resistance i3. Since the E. M. F.s applied to the grids of the modulator tubes I4 and I5 are respectively each 90 removed in phase from the voltage applied to the grid of the tube l2, the resultant plate current or current flowing thru the resistance I3"due to the dinerence in the currents thru the tubes I4 and I5 will be 90 different in phase from the current produced therein by the tube l2. The resultant plate current flowing thru the resistance l3 will either lead or lag the current therein due to the tube l2,

depending on the instantaneous polarity of the modulation applied to the screen'grids of the tubes i4 and i5. Hence the voltage developed across the resistance l3 which is the vector sum of the voltages produced by the current due to the tube l2 and the resultant current of the two modulator tubes II and I! will be alternately advanced and retarded in phase with respect to the phase of the voltage ofthe master oscillator.

The angle of the phase shift will be, within limits of the order of 30", directly proportional to the amplitude of the resultant current from. the modulator tubes I 4 and I5. By making the amplitude of this current inversely proportional to the frequency of the modulating current by means of the correction systems 22, 23 and multiplying the frequency a suflicient number of times by multiplier H! as described in my U. S. Patent #1,941.068 there will be produced a frequency modulated wave in the manner described in the aforementioned patent, but having the advantage of a uniform frequency characteristic and freedom from certain distortions which may be present in the system described in that patent. In the present system, it is unnecessary to introduce any phase-changing network in the output circuits of the modulator tubes. The output of the modulator tubes may be combined directly with the output of the carrier amplifier tube in a common aperiodic load. This removes the limitation which the phase-shifting network in the output of the modulator tubes of the old system may impose upon a uniform frequency characteristic.

I have described what I believe to be the best embodiment of my invention. I do not wish,

however, to be confined to theembodiment shown,

but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim: v 1. Apparatus for modulating the phase of a radio frequency carrier, comprising means for generating a radio frequency carrier current of fixed frequency, an amplifier, a balanced modulator, means for applying the radio frequency carrier current to the input circuit of said amplifier and said balanced modulator in such phase relation that the voltages applied to the inputs of the modulators are 180 out of phase with each other, and out of phase with the current applied to the input of the amplifier, and means for combining the output of the amplifier and modulator.

2. Apparatus for modulating the phase of a radio frequency carrier, comprising means for generating a radio frequency carrier current of fixed frequency, an amplifier, a balanced modulator, means for applying the radio frequency carrier current to the input circuit of said amplifier and said balanced modulator in such phase relation that the voltages applied to the inputs of the modulators are out of phase with each other, and 90 out of phase with the current applied to the input of the amplifier, and an aperiodic load for combining the output of the amplifier and modulator.

EDWIN H. ARMSTRONG. 25 

